The present invention relates to digital filtering of electronic signals, and more particularly to an improved finite impulse response (FIR) filter based upon squaring that uses the filter itself to determine a constant equal to the sum of the squares of the filter coefficients.
U.S. Pat. No. 5,561,616, issued to the present inventor on Oct. 1, 1996 entitled "FIR Filter Based Upon Squaring", illustrates a FIR filter based on squaring that reduces the complexity of the filter as opposed to standard FIR filter convolutions. At the output of the FIR filter is a subtracting circuit which has as one of the inputs a constant equal to the sum of the squares of the filter coefficients. The sum of the squares of the coefficients is indicated as being determined in a non-real-time manner with minimal hardware, by pre-calculation, or externally as an entry with each new set of coefficients as they are loaded.
Also once the set of coefficients is loaded into the filter, when the filter is coupled to a processor bus where multiple devices may access the filter, it is sometimes desirable to be able to determine what the coefficients of the filter are. A shadow set of values may be kept in a central processing unit (CPU) coupled to the bus from which the FIR filter was loaded, but this does not allow testing of the FIR filter by reading back the filter coefficients that were written. Or the coefficient inputs of the filter itself may be tied into a separate filter bus for reading back the filter coefficients, but this adds to the complexity of the FIR filter.
What is desired is an improved FIR filter based on squaring where the modification provides a way to easily determine the sum of the squares of the coefficients, while being accessible to external devices on a processor bus.